The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to a fixing device included in an image forming apparatus for fixing a toner image on a recording medium by using a halogen heater as a heat source.
A copier, for example electrostatically forms a latent image representative of a document image on a photoconductive element or image carrier, develops the latent image with developing means to thereby produce a corresponding toner image, and transfers the toner image to a paper sheet or similar recording medium. The copier then fixes the toner image on the paper sheet with a fixing device including a heat roller. The fixing device generally uses a halogen heater or similar radiation heater or radiation heat source. The radiation heater includes a glass tube accommodating a tungsten filament and filled with inactive gas, which is generally nitrogen, argon or krypton. This kind of fixing device is low cost, safe and long life and extensively used in various image forming apparatuses including a copier.
The above-described type of fixing device includes a heat roller and a press roller pressed against the heat roller. While a paper sheet is passed through a nip between the heat roller and the press roller, a toner image carried on the paper sheet is fixed by heat and pressure. The halogen heater is accommodated in the heat roller in order to heat the heat roller by radiating heat. This kind of heating system is generally referred to as an indirect heating system. In a direct heating system the heat roller is provided with a heat generating layer on its inner or outer periphery, so that the surface of the roller generates heat. The indirect heating system needs a longer period of time for the heat roller to be warmed up to a preselected fixing temperature than the direct heating system.
There has recently been developed an energy saving type of fixing device including a heat roller implemented by a tubular base that is formed of aluminum or iron and has a wall thickness as small as about 0.5 mm. This type of fixing device reduces the warm-up time of the heat roller to the fixing temperature even to about 10 seconds. Such a short warm-up time makes it needless to feed preheating current to the developing device even in a stand-by state. This, coupled with the fact that the fixing device can be turned off when not used, successfully saves energy. However, the warm-up time of the heat roller is longer than the warm-up time available with the direct heating system.
Japanese Patent Laid-Open Publication No. 11-174899 discloses a fixing device including a constant voltage circuit for reducing voltage variation. This fixing device uses heating means having a color temperature of 2,400 K or above.
More specifically, the halogen heater is filled with the previously mentioned inactive gas and a trace of halogen substance, e.g. iodine, bromine or chlorine. Usually, tungsten starts vaporizing at a temperature below its melting point and decreases in diameter little by little until it snaps. In the case of the halogen heater, tungsten vaporized from the fi lament repeatedly reacts with halogen gas confined in the glass tube and decomposes. Such a halogen cycle provides the halogen heater with necessary durability.
Today, a halogen heater not filled with a halogen substance or accommodating a carbon filament, which performs far infrared radiation, is under development from the environment standpoint.
The glass tube of the halogen heater is formed of quartz glass in order to withstand high temperature, which is necessary to maintain the halogen cycle. Quartz is either transparent quartz made from crystal or semitransparent quartz made from silica. A tube formed of semitransparent quartz is low in transparency, but low cost and equivalent with transparent quarts as to other physical properties. A semitransparent quartz tube is therefore usually applied to the halogen heater that does not need a precise optical characteristic. The semitransparent quartz tube has a transmission of about 80% with respect to light having a wavelength of 300 nm to 3,000 nm. Generally, a conventional semitransparent quartz tube has an outside diameter of 6 mm to 10 mm and a wall thickness of 1.0 mm to 1.2 mm.
A relation between the heat radiation from the halogen heater having the above-described specification and losses has generally been grasped as experimental values in the steady state, i.e., at the fixing temperature. Specifically, it is generally understood that infrared radiation to the inner surface of the heat roller is about 86%, visible radiation is about 7%, a terminal loss is about 2%, and a loss ascribable to the glass tube is about 5%.
The problem with the indirect heating type of fixing device is that the warm-up of the heat roller to the fixing temperature is slow, as stated earlier. If the warm-up of the heat roller can be accelerated, it is possible to enhance the manipulability of the fixing device or an image forming apparatus using it and to promote energy saving while preserving the various advantages of the indirect heating system.
Generally, the warm-up time of the fixing device using a heat roller is dependent mainly on the thermal capacity of the heat roller, which is a member to be heated. To reduce the warm-up time, it has been customary to reduce the diameter or the wall thickness of the heat roller. However, this kind of scheme reduces the rigidity of the heat roller and makes it impossible to reduce the thermal capacity beyond a certain limit while maintaining the minimum mechanical strength.
As a result of analysis on why the warm-up of the fixing device using a halogen heater is slow, there were found the following causes (1) and (2).
(1) A substantial period of time is necessary for the halogen heater itself to reach a filament temperature of 2,500 K at which radiation is becomes stable. The warm-up time of a 100 V, 1,200 W halogen heater is as long as 1 second or more. The temperature elevation of the heat roller is delayed by such a period of time. The warm-up time of the filament itself increase in proportion to the thermal capacity thereof. More specifically, as the diameter and length of the filament increase, the thermal capacity of the filament increases, extending the warm-up time of the filament.
(2) In principle, no losses occur if the entire energy input to the halogen heater is radiated from the filament and then radiated from the inner surface of the heat roller to become heat. In practice, however, the gas around the filament absorbs the heat of the filament due to convection thereof. Further, when light issuing from the filament is transmitted through the glass tube, the glass tube absorbs part of the light. Experiments showed that at the time of warm-up the glass tube and gas confined therein absorbed about one-fourth of the radiation from the filament, allowing only three-fourths of the radiation to be radiated to the inner surface of the heat roller.
The influence of the glass tube and gas confined therein is particularly noticeable in a fixing device of the type causing substantially no radiation to occur from the glass tube to the heat roller and having a short warm-up time, as will be described specifically later. The loss ascribable to the glass tube of the halogen heater is generally considered to be about 5% of the entire radiation and technically unavoidable because of such a low ratio. This ratio, however, holds only in the steady state in which the temperature of the halogen heater is stable. In an energy saving type of fixing device that warms up the heat roller rapidly, the ratio of the loss ascribable to the glass tube during warm-up is as great as about 25%, as determined by experiments. This suggests that there is sufficient room for technical improvement as to the warm-up time of the fixing device using a halogen heater.
The warm-up time to the fixing temperature is generally several 10 seconds. In this sense, a period of time of 1.7 seconds necessary for the radiation heater itself to be warmed up just after the turn-on of a power source may not be long. However, in the energy saving type of fixing device whose warm-up time to the fixing temperature is as short as about 10 seconds, the warm-up time of the radiation heater itself just after the turn-on of the power source is not negligible.
Another problem with the conventional halogen heater is that its response at the time of turn-on and turn-off is slow and brings about the temperature ripple of the heat roller when a paper sheet arrives at the fixing device. Rush current that flows when the power source is turned on is still another problem particular to the halogen heater.
Technologies relating to the present invention are also disclosed in. e.g., Japanese Patent Laid-Open Publication Nos. 7-121041, 7-254393, 9-265246 and 11-174899.
It is another object of the present invention to provide a fixing device using a halogen heater achieving a short warm-up time and saving energy, and an image forming apparatus including the same.
In accordance with the present invention, a fixing device includes a heat roller accommodating a halogen heater that has a glass tube filled with inactive gas and a halogen substance, and a press roller pressed against the heat roller. The glass tube has a mean transmission of 94% with respect to light having a wavelength of 300 nm to 3,000 nm.
Also, in accordance with the present invention, an image forming apparatus includes a fixing device including a heat roller accommodating a halogen heater that has a glass tube filled with inactive gas and a halogen substance, and a press roller pressed against the heat roller. The glass tube has a mean transmission of 94% with respect to light having a wavelength of 300 nm to 3,000 nm.